The present invention relates to a method and an apparatus for gasifying carbonaceous material.
More particularly, the present invention relates to a method of gasifying carbonaceous material, in which carbonaceous material is gasified to produce a product gas in a gasification reactor of a gasification system. The product gas and ash entrained therewith, residual carbon, and gaseous tar compounds are discharged from the gasification reactor to a product gas channel. There, the product gas is cooled in a gas cooler, whereby the tar compounds condense into a liquid form, which tends to stick on surfaces. Solid material containing ash particles and residual carbon is separated from the gasification system and guided to an ash reactor. Oxygen-containing gas is supplied to the ash reactor, whereby the residual carbon reacts with oxygen to generate ash particles and exhaust gas.
The invention also relates to an apparatus for gasifying carbonaceous material. The apparatus comprises a gasification system including a gasification reactor, a product gas channel connected to the gasification reactor, a gas cooler arranged in the product gas channel, and means for separating solid material containing ash particles and residual carbon from the gasification system. The apparatus also includes an ash reactor having means for treating residual carbon in the ash reactor with oxygen, and means for supplying the solid material separated from the gasification system to the ash reactor.
When gasifying carbonaceous fuel, air and/or oxygen as well as steam are supplied to the gasification reactor, whereby the aim is to generate product gas mainly consisting of carbon monoxide (CO) and hydrogen (H2).
To bring about endothermic reactions generating carbon monoxide and hydrogen, heat must be either released from the fuel by means of partial combustion or it must be brought to the gasification reactor in the form of an external heat exchange medium. When the gasification is not complete, a portion of the carbon in the fuel exits with the product gas as non-gasified char. The gasification temperature, especially with fluidized bed gasifiers, is often relatively low, e.g., 500-1000° C., such that the non-gasified carbon may significantly reduce the gas production level of the gasifier.
The product gas leaving the gasification reactor generally contains ash particles, which must be removed, for example, by a particle filter, before further processing the product gas. Since particle filters for gas operating at a high temperature are expensive and are prone to be damaged, the product gas is usually cooled prior to filtering. Especially, when gasifying waste material and biomass, significant amounts of tar compounds may be generated, which are gaseous at the gasification temperature, but condense at lower temperatures into sticky drops and even into solid particles. The condensed tar compounds may, for example, form deposits on the heat exchange surfaces of the gas cooler and on the filter, which are difficult to remove. Thus, the tar compounds decrease the heat exchange capacity of the heat exchange surfaces and clog filtering elements of the filter, thereby increasing the pressure loss caused by the filter.
U.S. Pat. No. 5,658,359 discloses a method in which heat exchange surfaces of a gas cooler in a fluidized bas gasifier are mechanically cleaned of deposits by passing bed sand, limestone, or a material separated from the product gas by a particle filter downstream of the gas cooler, to the heat exchange surfaces.
U.S. Pat. No. 4,613,344 discloses a method in which the sticking of impurities of the product gas is prevented by quickly cooling the gas through critical temperature zones. Cooling of the gas is accelerated by adding an inert material, e.g., aluminiumoxide (Al2O3), to the product gas in the gas cooler, which material is separated from the product gas downstream of the gas cooler by a centrifugal separator, cooled in a heat exchanger fluidized by the product gas, and recirculated to the product gas.
Solid material separated from the product gas, e.g., by a particle filter, may contain, in addition to ash, a considerable amount of char. The fly ash and bottom ash of the gasification reactor may also contain PAH compounds and other carbon compounds harmful to the environment. Thus, the ash removed from the gasification system must usually be post-treated before it may be gathered to public landfill sites or utilized as an industrial or agricultural raw material, for example.
International Patent Publication No. WO 00/43468 discloses a method in which carbonaceous solid material collected from the product gas filter of a fluidized bed gasifier is oxidizing in another fluidized bed reactor, and the oxygen-containing gas discharging from the reactor is guided to the fluidized bed gasifier to act as a secondary gasification gas.
U.S. Pat. No. 4,347,064 discloses a method in which partially gasified material collected from the separators of a circulating fluidized bed gasifier is brought to the final gasification in another gasifier, the product gas of which is supplied to act as a fluidization gas in a circulating fluidized bed gasifier.